Size matters

Scientists know that large carnivores, such as wolves, are vulnerable to changes in their habitat. Likewise, they know that small herbivores, such as mice, tend to thrive in human-altered environments. However, understanding why this is so and predicting how fragmented habitats will affect a wide range of species of varying body size remains a formidable challenge. Recently, several researchers developed a new computer model which focuses on body size to predict how various groups of organisms could be affected by changes in the environment.

Building on previous studies, John Haskell (Utah State University) and colleagues Mark Ritchie (Syracuse University) and Han Olff (Wageningen University), looked at how animals forage for food based on their body size and how the food is distributed in the environment. Their model predicts home range--the area an animal uses throughout its life--as a result of body size and the structure of the environment.

"What we found is that an organism's body size is a key determinant of how often it can be expected to encounter food in its environment," says Haskell. "Resource density is often scale-dependent, that is, it changes with the scale of measurement. The food density you see at one square meter changes when you zoom out to one square kilometer."

The researchers assert that measurement scale is related to body size. Haskell explains that bigger animals measure the environment with bigger 'rulers.' While a mouse might look at the world one foot at a time, a moose can look at an acre at a time. Because these animals perceive their environment at different scales, they see different amounts of resources. In general, the researchers assert, food density decreases with increasing body size.

Haskell elaborates: "If I walk out into the meadow below my house, I find a high density of grass. But as I move up out of the meadow into the surrounding sagebrush, the grass density gets much lower. A mouse might never leave the meadow, but a moose has to get food from my meadow and the one in the next drainage. So to the moose, the density of food seems to be lower because there is a ridge of less edible sagebrush in between. The moose is averaging over areas with high and low grass density while the mouse is only counting areas of high density. This is why large animals have bigger home ranges than we would expect based on their metabolic rates alone."

Not only does the model explain the differences among the home ranges of herbivorous and carnivorous mammals and birds, says Haskell, but it also predicts that the home range of an animal can be expected to increase with habitat fragmentation.

Furthermore, as has already occurred with such large species as the American Condor, the model predicts that the home ranges of larger species should be much more sensitive to habitat fragmentation than those of smaller species.

Says Haskell: "We thought: Wow! The model gives us the reason that large animals, especially carnivores, are so vulnerable to habitat changes. Carnivores' food is naturally very sparse and patchy. Further fragmentation makes it very difficult for these animals to obtain enough food and survive in fragmented environments."

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The oral paper, "Home Range Scaling Fractal Environments" will be held as part of Oral Session #9, Theoretical Ecology--Populations, Interactions, from 8 AM - 11:30 AM on Monday, August 5, 2002 in the Grand Ballroom Central, Radisson Hotel.

To find out more about the Ecological Society of America's 87th Annual Meeting, please visit our website http://www.esa.org/Tucson. Held at the Tucson Convention Center August 5-9, 2002, these sessions are part of a gathering of over 3,500 scientists and conservationists.

The Ecological Society of America (ESA) is a scientific, non-profit, 7,800-member organization founded in 1915. Through ESA reports, journals, membership research, and expert testimony to Congress, ESA seeks to promote the responsible application of ecological data and principles to the solution of environmental problems. ESA publishes three scientific, peer-reviewed journals: Ecology, Ecological Applications, and Ecological Monographs. Information about the Society and its activities is published in the Society's quarterly newsletter, ESA NewSource, and in the quarterly Bulletin.

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